WASHINGTON (AFP) - Fifty senior Republican national security officials issued a stinging rejection of their party's White House nominee Donald Trump on Monday, warning he would be "the most reckless president in American history" if elected.

The group, some of whom had already announced they would not vote for Trump, included former homeland security chiefs, intelligence directors, senior presidential advisors and a former US trade representative. They served under Republican presidents from Richard Nixon to George W. Bush.

"We are convinced that he would be a dangerous president and would put at risk our country's national security and well-being," they wrote in a statement published in The New York Times.

While they did not say they would vote for Democrat Hillary Clinton - indeed they expressed "doubts" about her - they were clear in stating that "none of us will vote for Donald Trump."

The experts essentially declared the brash billionaire unfit for office, echoing Clinton's criticism by saying that Trump "lacks the character, values, and experience to be president" and displays "alarming ignorance of basic facts" of international politics.

The New York real estate mogul also has shown no willingness to learn about foreign affairs or national security threats, "acts impetuously" and lacks self control, the statement said.

"He is unable or unwilling to separate truth from falsehood," the group wrote, saying Trump possesses a set of "dangerous qualities" that should disqualify him from the presidency.

"We are convinced that in the Oval Office, he would be the most reckless president in American history." They warned that the political neophyte's "erratic behavior" has alarmed America's closest allies and that he fails to recognise the indispensible nature of such diplomatic relationships.

The signatories include Tom Ridge and Michael Chertoff, the first and second homeland security secretaries under president George W. Bush, former director of National Intelligence John Negroponte and Bush-era CIA director Michael Hayden.

Also in the group are former US Trade Representative and World Bank president Robert Zoellick and Eric Edelman, former national security advisor to vice president Dick Cheney.

The group joins members of the security establishment who have already come out against Trump, including former CIA director Michael Morell, who last Friday accused Trump of being an "unwitting agent" of Russian President Vladimir Putin.